This application relates to compensation for optical dispersion, and more specifically, to techniques for reducing polarization-mode dispersion in optical media such as optical fiber links in optical wavelength-division multiplexing (WDM) systems.
Some optical transmission media such as optical fibers are birefringent and hence exhibit different refractive indices for light with different polarizations along two orthogonal principal directions. Therefore, an optical signal, that comprises two components along the two orthogonal principal directions for each frequency, can be distorted after propagation through the transmission medium since the two components propagate in different group velocities. This optical dispersion is generally referred to as polarization-mode dispersion (xe2x80x9cPMDxe2x80x9d).
The degree of PMD may be approximately characterized by the average differential group delay (xe2x80x9cDGDxe2x80x9d) between two principal states of polarization. Typical causes for such birefringence in fibers include, among others, imperfect circular core and unbalanced stress in a fiber along different transverse directions. The axis of birefringence of the optical fiber can change randomly on a time scale that varies between milliseconds and hours, depending on the external conditions. Thus, the DGD in an actual PMD fiber is not a fixed value but a random variable that has a Maxwellian probability density function.
Such polarization-mode dispersion is undesirable in part because the pulse broadening can limit the transmission bit rate, the transmission bandwidth, and other performance factors of an optical communication system. In fact, PMD is one of key limitations to the performance of some high-speed optical fiber communication systems at or above 10 Gbits/s per channel due to the fiber birefringence. Fibers with significant PMD (e.g., about 1 to 10 ps/km1/2) are used in various fiber networks, particularly in those that were deployed in 1980""s and early 1990""s.
The present disclosure includes PMD compensation in WDM systems without wavelength demultiplexing. All WDM channels are guided through the same optical path in a PMD compensation module. The overall system performance can be improved by reducing the fading probability for one or more worst-performing channels at any given time.
One embodiment of a PMD compensation system includes a PMD compensator and a feedback control that produces a control signal for controlling the PMD compensator. The PMD compensator guides a plurality of WDM optical channels through a common optical path to modify a differential group delay (DGD) in each channel. The PMD compensator is operable to adjust the DGD in response to the control signal. The feedback control is coupled to measure a property of the channels as a whole and operable to generate the control signal according to the measured property.